[en] Self-fluorescence measurements have two disadvantages: the excitation and relaxation mechanisms are not understood, therefore the fluorescing level is difficult to assign, and fluorescence from daughter ions is also present. To obtain the 5f/sup n/ ion energy level structure with the highest possible precision, ions dilutely substituted into single crystal host lattices are needed. Concentrated systems such as EsBr₃ will exhibit line broadening due to ion-ion interactions. This limits the usefulness of any pure system. The use of dye laser techniques to study the energy level structure of the heavier actinides in single crystals of LaCl₃ is discussed